Lignin, a complex phenolic polymer, is a major component in cell walls of secondary xylem. In general, lignin constitutes 25% of the dry weight of the wood, making it the second most abundant organic compound on earth after cellulose. Although lignin contributes to the strength and rigidity of the stem, and protects microfibrils from physical, chemical and biological attack, it hinders the process of converting wood into paper. In order to liberate wood fibers for the production of paper, most of the lignin must be removed from the processed wood chips. Extracting lignin from wood fibers is a difficult and expensive process, involving harsh chemicals and yielding toxic waste products.
Consequently, practitioners have searched for more cost-effective and environmentally-friendly methods of reducing the lignin content in wood products. One alternative involves genetically modifying the biosynthetic pathway of lignin. For example, Chiang et al. have attempted to reduce the lignin content in a plant by genetically modifying the plant's monolignol biosynthetic pathway. See WO 02/20717. The method involved transforming a plant with multiple genes from the phenylpropanoid pathway, including key lignin control sites in the monolignol biosynthetic pathway such as the enzymes 4-coumarate-CoA ligase (4CL), coniferyl aldehyde 5-hydroxylase (Cald5H), S-adenosyl-L-methionine (SAM)-dependent 5-hydroxyconiferaldehyde, O-methyltransferase (AldOMT), coniferyl alcohol dehydrogenase (CAD) and sinewy alcohol dehydrogenase (SAD). Meanwhile, others have attempted to reduce lignin content by individually introducing copies of these genes into plant genomes. See e.g. WO 00/58489 (Scald); WO 99/24561 (4CL). Practitioners also have employed these genes in antisense strategies to modulate lignin biosynthesis. See e.g. WO 99/24561. While some of these methods successfully down-regulated lignin synthesis, the down-regulation of lignin can be detrimental to plant phenotype. Anterola et al., Phytochemistry, 61:221-294 (2002). Thus, improved methods for modulating lignin expression are needed.
A recent method of silencing gene expression at the mRNA level has emerged as a powerful alternative to prior technologies. RNA interference (RNAi) is a post-transcriptional process triggered by the introduction of double-stranded RNA (dsRNA) which leads to gene silencing in a sequence-specific manner. The initial discovery of RNA interference in C. elegans (Fire et al., Nature, 391:806-811 (1998) and U.S. Pat. No. 6,506,559) has been followed by numerous examples of organisms where introduction of dsRNA can induce the sequence-specific silencing effect. For example, RNAi has been reported to occur naturally in organisms as diverse as nematodes, trypanosmes, plants, fungi and animals. In nature, RNAi most likely serves to protect organisms from viruses, modulate transposon activity and eliminate aberrant transcription products.
Studies in the fruit fly Drosophila melanogaster suggest that RNAi is a two-step mechanism (Elbashir et al., Genes Dev., 15(2): 188-200 (2001)). First, long dsRNAs are cleaved by an enzyme known as Dicer into 21-23 ribonucleotide (nt) fragments, called small interfering RNAs (siRNAs). Then, siRNAs associate with a ribonuclease complex (termed RISC for RNA Induced Silencing Complex) which target this complex to complementary mRNAs. RISC then cleaves the targeted mRNAs opposite the complementary siRNA, which makes the mRNA susceptible to other RNA degradation pathways.
RNAi may offer an alternative to prior methods of controlling lignin synthesis. Before the potential can be realized, however, DNA constructs that can initiate RNAi processes in the context of lignin synthesis must be developed.